U.S. patent number 4,813,421 [Application Number 07/123,457] was granted by the patent office on 1989-03-21 for oxygen sensing pacemaker.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Michael D. Baudino, Gene A. Bornzin, Dennis A. Brumwell, Michael D. De Franco, Joseph F. Lessar, Jeffrey A. Schweitzer.
United States Patent |
4,813,421 |
Baudino , et al. |
March 21, 1989 |
Oxygen sensing pacemaker
Abstract
A heart pacemaker including a two wave length reflectance
oximeter for determining oxygen saturation. The pacing rate is
increased or decreased in response to the measured oxygen
saturation. By appropriate multiplexing and timing functions, the
two wave length reflectance oximeter is included in a pacing lead
coupled to the pacemaker which requires only three conductors.
Inventors: |
Baudino; Michael D. (Coon
Rapids, MN), De Franco; Michael D. (Blaine, MN), Lessar;
Joseph F. (Coon Rapids, MN), Brumwell; Dennis A.
(Bloomington, MN), Bornzin; Gene A. (Camarillo, CA),
Schweitzer; Jeffrey A. (Minneapolis, MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
|
Family
ID: |
26821586 |
Appl.
No.: |
07/123,457 |
Filed: |
November 19, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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896695 |
Aug 15, 1986 |
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Current U.S.
Class: |
600/333; 356/41;
607/22 |
Current CPC
Class: |
A61N
1/056 (20130101); A61N 1/36557 (20130101) |
Current International
Class: |
A61N
1/05 (20060101); A61N 1/365 (20060101); A61B
005/00 () |
Field of
Search: |
;128/633,634,419PG,666
;356/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Article entitled "An Integrated Circuit-Based Optical Sensor for in
vivo Measurement of Blood Oxygenation", authored by Joseph M.
Schmitt et al, published in IEEE Transactions on Biomedical
Engineering, vol. BME-33, No. 2 Feb. 1986. .
Article entitled "A Proposed Minature Red/Infrared Oximeter
Suitable for Mounting on a Catheter Tip", authored by Sinclair Yee
et al, published in IEEE Transactions on Biomedical Engineering,
vol. BME-24, No. 2, 1977. .
Article entitled "A Light Emitting Diode Skin Reflectance
Oximeter", authored by A. Cohen et al, published in Medical and
Biological Engineering, vol. 10, pp. 385-391, 1972. .
Article entitled "The Theory and Construction of Oximeters",
authored by W. J. Reichert, published in Int. Anesth. Clin., vol.
44, Spring 1966. .
Article entitled "A New Instrument for Rapid Measurement of Blood
Oxygen Saturation and Hb Concentration", author unknown, published
in Biomedical Engineering, vol. 5, 1970. .
Article entitled "The Principle Design and Features of a New
Hb-Oximeter", authored by F. J. Janssen, published in Medical &
Biological Engineering, vol. 10, pp. 9-22, 1972..
|
Primary Examiner: Picard; Leo P.
Assistant Examiner: Harrison; Jessica J.
Attorney, Agent or Firm: Duthler; Reed A. Breimayer; Joseph
F. Rooney; John L.
Parent Case Text
This is a divisional of co-pending application Ser. No. 896,695
filed on Aug. 15, 1986.
Claims
In conjunction with the above written specification and drawings,
we claim:
1. An implantable two wavelength reflectance oximeter
comprising:
a sensor capsule comprising a first light emitting diode emitting
light of a first wavelength, a second light emitting diode emitting
light of a second wavelength and a semiconductor means for
regulating flow of current dependent upon the amount of light
impinging on said semiconductor means;
first and second conductors electrically coupled to said first and
second light emitting diodes such that first and second light
emitting diodes are electrically coupled between said first and
second conductors but at opposite polarities, said second conductor
electrically coupled to said semiconductor means;
third conductor electrically coupled to said semiconductor means
such that said semiconductor means is electrically coupled between
said second and third conductors;
first voltage generating means for generating a voltage
differential between said first and second conductors of a first
polarity and for simultaneously generating a voltage differential
between said second and third conductors;
second voltage generating means for generating a voltage
differential between said first and second conductors of a second
polarity and for simultaneously generating a voltage differential
between said second and third conductors; and
current measuring means coupled to said second and third conductors
for measuring current flow through said semiconductor means.
2. An implantable oximeter according to claim 1 further comprising
means for sequentially activating said first voltage generating
means, deactivating said first voltage generating means, activating
said second voltage generating means, and deactivating said second
voltage generating means.
3. An implantable oximeter according to claim 2 further comprising
means for activating said current sensing means only while said
first and second voltage generating means are activated.
4. An implantable oximeter according to claim 3 wherein said means
for activating said current sensing means activates said current
sensing after activation of said first voltage generation means and
deactivates said current sensing means prior to activation of said
second voltage generating means and wherein said current sensing
activating means activates said current sending means after
activation of said second voltage generation means and deactivates
said current sensing means while said second voltage generating
means is activated.
5. An implantable oximeter according to claim 1 or claim 2 or claim
3 or claim 4 above further comprising a stimulating electrode and a
pulse generator for generating stimulating pulses to be applied to
said stimulating electrode.
6. An implantable oximeter according to claim 5 above further
comprising:
means for enabling said sequential activating means following
generation of said stimulating pulse.
7. An implantable oximeter according to claim 6 above further
comprising means for sensing electrical activity of the body and
wherein said enabling means enables said sequential activating
means following sensing of electrical activity by said sensing
means.
8. An oximeter according to claim 2 further comprising:
a first electrode coupled to said first conductor;
a second electrode, insulated from said first conductor;
pulse generator means coupled to said first conductor and to said
second electrode for generating a stimulating pulse across said
first and second electrodes.
9. An implantable oximeter according to claim 8 further comprising
enabling means coupled said sequential activating means for
enabling said sequential activating means after generation of a
stimulating pulse across said first and second electrodes.
10. An oximeter according to claim 8 or claim 9 further comprising
voltage follower means coupled to said second electrode for causing
the voltage on said second electrode to follow the voltage on said
first conductor whereby no voltage differential exists between said
first and said second electrode during activation of said first and
second voltage generating means.
11. An implantable oximeter according to claim 9 further comprising
oscillator means for activating said pulse generator means at the
expiration of an escape interval period, said oscillator means
reset concurrent with generation of a stimulating pulse across said
first and second electrodes.
12. An implantable oximeter according to claim 11 further
comprising means for sensing electrical activity of the body,
coupled to said oscillator means and for resetting said escape
interval period of said oscillator means.
13. An oximeter according to claim 12 above wherein said sensing
means is further coupled to said enabling means and wherein said
enabling means enables said sequential activating means following
sensing of electrical activity of the body by said sensing
means.
14. An oximeter according to claim 11 above further comprising
adjustment means coupled to said oscillator means for adjusting the
length of said escape interval, said adjustment means further
coupled to said current sensing means for adjusting said escape
interval in response to sensed current passing through said
semiconductor means.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to medical devices and more
specifically relates to implantable electronic devices for muscle
stimulation.
The earliest implantable pacing systems operated asynchronously to
normal physiologic functions. U.S. Pat. No. 3,057,356, issued to
Greatbatch, teaches such a pacemaker which includes a fixed rate
oscillator which determines an escape interval. At the expiration
of each escape interval, a pacing pulse is generated. Subsequent
designs, such as in U.S. Pat. No. 3,478,746 issued to Greatbatch,
incorporated a sense amplifier coupled to the pacing electrode,
which sensed the electrical activity indicative of a contraction of
the heart and reset the oscillator to restart timing the escape
interval. These pacemakers, called demand pacemakers, only paced if
no natural contractions were sensed within the escape interval.
As an alternative to regulating pacing rate by sensing the
contractions of the heart, some pacemakers have regulated rate in
response to measurement of some other, physiological parameter.
There have been pacemakers proposed which vary rate in accordance
to instantaneous blood pressure within the right atrium, as in U.S.
Pat. No. 3,358,690, in response to respiration as in U.S. Pat. No.
3,593,718, in response to physical activity as in U.S. Pat. No.
4,140,132 or in response to neurological activity as in U.S. Pat.
No. 4,210,219. The most promising techniques appear to involve
varying of pacing rate in response to sensing of chemical
parameters of the blood. For example, U.S. Pat. Nos. 4,009,721 and
4,252,124 teach pacemakers in which an implantable pH sensor
determines the rate of the pacing oscillator. U.S. Pat. No.
4,202,339 issued to Wirtzfeld and U.S. Pat. No. 4,399,820 issued to
Wirtzfeld et al, both teach a pacing system in which the rate of an
asynchronous pacing oscillator is controlled by the oxygen level of
the intracardiac venous blood. U.S. Pat. No. 4,467,807 issued to
Bornzin combines the techniques of varying the rate of the
pacemaker in response to sensed oxygen with the demand function, so
that an interaction of both of these factors determines the
delivery of pacing impulses by the pacemaker.
SUMMARY OF THE INVENTION
The present invention comprises an improved oxygen sensor and
associated circuitry for use within a cardiac pacemaker of the type
in which pacing rate is dependent upon the percentage of oxygen
saturation of the intracardiac venous blood. The present invention
includes a hermetically sealed sensor capsule containing a two
wavelength reflectance oximeter. The method of construction of this
capsule assures its suitability for long term human implant. In
addition, the invention includes timing, processing and output
circuitry for operating the sensor in a predetermined time
relationship with pacing output pulses and which allows the
construction of a long term implantable lead which performs EKG
sensing, cardiac pacing and two wave length reflectance oximetry
using only three conductors. Minimizing the number of conductors is
believed particularly valuable in long term implantable devices,
where experience has shown that electronic complexity is preferable
to mechanical complexity.
The invention will be more readily and easily understood in
conjunction with the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a prior art rate adaptive demand
pacemaker in which sensed oxygen saturation is used to regulate the
pacing rate.
FIG. 2 is a plan view of a pacing and oxygen sensing lead according
to the present invention.
FIG. 3 is a side sectional view of the sensor body, a subassembly
of the sensor capsule.
FIG. 4 is a side sectional view of the window assembly, a
subassembly of the sensor capsule.
FIG. 5 is a side sectional view through the completed sensor
capsule.
FIG. 6 is a top sectional view through the completed sensor
capsule.
FIG. 7 is a functional diagram of the sensor and associated
circuitry.
FIG. 8 is a circuit diagram of the analog circuitry associated with
the sensor.
FIG. 9 is a schematic of the digital timing circuitry associated
with the sensor capsule.
FIG. 10 is a timing diagram illustrating the operation of the
sensor capsule, and its relation to the operation of the cardiac
pacemaker with which it is intended to be employed.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a rate adaptive demand pacemaker in which sensed
oxygen level is used to vary the escape interval of a demand
pacemaker, as disclosed in U.S. Pat. No. 4,467,807 issued to
Bornzin, and incorporated herein by reference in its entirety. This
figure is intended both to illustrate pertinent prior art in this
area and to provide a general illustration of how the circuitry and
sensor of the present invention would be functionally related to
the circuitry of an implantable pacemaker.
FIG. 1 shows a single chamber pacing system including a sensor 12
within the right ventricle 20 of heart 10, which generates a
voltage on line 14 proportional to percentage of oxygen saturation
in the right ventricle of the heart. The voltage on line 14 is
processed by sensor processing circuitry 30 which includes an
analog to digital converter which converts the voltage on line 14
to a binary number which is used to control the pacing rate set by
demand logic 106 and fixed rate oscillator 112. Line 104 is coupled
to an electrode in right ventricle 20 of the heart 10 and delivers
electrical signals to the sense amp 102 which generates an output
on line 108 indicative of the sensing of a natural contraction of
the human heart. On sensing such a contraction, the timing period
of the demand logic is reset. On expiration of the escape interval
determined by demand logic 106 and fixed rate oscillator 112 under
the control of sensor processing 30, a signal is generated on line
34 which triggers driver 26 to produce a stimulating pulse via line
16, to be applied to an electrode 18 within the right ventricle of
the heart 10. A detailed description of the circuitry employed in
sensor processing block 30 and demand logic 106 can be found in
U.S. Pat. No. 4,467,807 at column 3 lines 59-68, column 4, lines
1-68 and column 5, lines 1-30.
Although the incorporation of the present invention in an
implantable pacemaker is discussed specifically with reference to
the pacemaker disclosed in FIG. 1, the invention is believed
equally applicable to other pacemakers employing alternate
circuitry configurations for determining rate in response to an
analog signal indicative of oxygen saturation. For example, it is
believed the invention would also be useful in conjunction with a
pacemaker as disclosed in U.S. Pat. No. 4,202,339 issued to
Wirtzfeld, cited above.
FIG. 2 is a plan view of a permanently implantable lead according
to the present invention. Lead 200 is provided with a pacing
electrode 210 at its extreme distal tip, which is held in place
within the heart by tines 212, which may be of the type described
in U.S. Pat. No. 3,902,501 issued to Citron et al or U.S. Pat. No.
4,269,198 issued to Stokes and incorporated herein by reference in
their entirety. Located proximal to electrode 210 is sensor capsule
214, which contains a two wavelength reflectance oximeter. Capsule
214 is spaced from electrode 210 by an insulative lead body 216
which encloses a conductor which couples electrode 210 to sensor
capsule 214. Proximal to sensor 214 is an elongated lead body 218
which is preferably sheathed in a pliant, insulative material such
as polyurethane or silicone rubber. Lead body 218 contains three
coaxially mounted coiled conductors coupled to sensor capsule 214
and extending to connector assembly 220. Connector assembly 220
includes three connector surfaces 222, 224, and 226 which are
coupled to the three conductors mounted within lead body 218. An
anchoring sleeve 228 is optionally included to stabilize lead 200
at the point of venous insertion. Anchoring sleeve 228 may be of
the type described in U.S. Pat. No. 4,437,475 issued to White.
FIG. 3 is a side sectional view of the sensor body assembly, a
subassembly of the sensor capsule 214 of the present invention. The
sensor body assembly includes a machined titanium sensor body 230,
having longitudinal surface 231 which serves to mount the ceramic
substrate 232 upon which the various electronic elements of the two
frequency reflectance oximeter are located. These elements include
a phototransistor 234, an infrared light emitting diode 236 and a
red light emitting diode 238. Diodes 236 and 238 are separated from
phototransistor 234 by a semicircular wall 240. At the proximal end
of sensor body 230 is an aperture 242 in which a sapphire or glass
feedthrough 246 is located. Feedthrough 246 has been brazed to
sensor body 230 around its entire circumference, using a braze such
as gold. Extending through and also brazed to feedthrough 246 are
two wires, of which only one, labeled 248, is visible in this view.
Wire 248 is soldered to a conductive pad located on ceramic 232. At
the distal end of sensor body 230 is a bore 250 which serves as a
mounting point for a coiled conductor.
FIG. 4 is a side sectional view of the window assembly of the
sensor capsule. The window assembly includes an optically pure
sapphire or glass tube 252 and two welding collars 254 and 256
located at the proximal and distal ends, respectively, of tube 252.
Welding collars 254 and 256 are fabricated of titanium and are
provided with circumferential indentations at their distal and
proximal ends, respectively, which receive the ends of sapphire
tube 252. Welding collars 254 and 256 are brazed to the proximal
and distal ends of sapphire tube 252 around their entire
circumference, using a braze which is preferably gold. Prior to
brazing, the proximal and distal ends of tube 252 are coated with a
thin film of niobium or similar metalization to facilitate brazing.
Vapor deposition or sputtering techniques may be used to provide
this thin film.
The capsule itself is assembled by sliding the window assembly of
FIG. 4 proximally over the body assembly of FIG. 3, until the
proximal end 258 of weld collar 254 is adjacent the shoulder 243 of
sensor body 230 (FIG. 3). The inner diameter of the proximal
portion 260 of weld collar 256 is the same as the outer diameter of
the distal portion 310 of sensor body 230. After assembly, weld
collar 254 and weld collar 256 may conveniently be laser welded to
sensor body 230, around the circumference of proximal end 258 of
weld collar 254 and of distal end 260 of weld collar 256. The
circular cross sections of weld collars 254 and 256 and of sensor
body 230 at the weld points simplifies the weld process by allowing
the assembly to simply be rotated under the welding beam. Laser
welding is the preferred method, but other methods such as electron
beam welding may also be appropriate. This procedure produces a
hermetically sealed sensor capsule appropriate for a long term
implant within the human body. The use of titanium for both the
sensor body 230 and for the weld collars 254 and 256 prevents any
corrosion at the weld. The interrelation of the sensor capsule to
the remainder of the components of the lead is described below in
FIGS. 5 and 6.
FIG. 5 is a side sectional view of the sensor capsule, mounted in
the lead of FIG. 2. Within bore 250 at the distal end of sensor
body 230 is located a coiled conductor 262 which is held in place
by crimps 264 which mechanically and electrically couple it to
sensor body 230. Conductor 262 extends to and is electrically
coupled with tip electrode 210 (FIG. 2).
At the proximal end of the sensor body is a capsule adapter 268,
which is laser welded at its distal end to sensor body 230 and
welding collar 254. Capsule adapter 268 is generally cylindrical
and hollow in its construction, and includes a cross bore 270 for
addition of backfill material 271. Welded to capsule adapter 268 is
a coiled conductor 272, which is coupled to connector pin 226 (FIG.
2). Located within the internal cavity 269 of adapter 268 are two,
mutually insulated coaxially coiled conductors including coiled
conductor 274 and coiled conductor 276, which are insulated by
insulating sheaths 278 and 280, respectively. Conductor 276 is
attached to wire 248, within a welding sleeve, not visible in this
illustration. Conductor 274 is coupled to a second wire, passing
through feedthrough 246. The entire lead is sheathed in an outer
insulative sheath 282, which is preferably fabricated of
polyurethane or other transparent, non-thrombogenic material.
Cavity 269 is backfilled with epoxy 271. All other labeled elements
correspond to identically numbered elements in FIGS. 3 and 4,
above.
FIG. 6 is a top cut away view of the sensor capsule mounted in the
lead of FIG. 2. In this view, the connection of conductors 274 and
276 to wires 248 and 247 is clearly visible. Conductors 274 and 276
are preferably multifilar coils which are welded to wires 247 and
248, and mounted within welding sleeves 284 and 286. In this view,
it can be seen that wires 247 and 248 are coupled to two metallic
pads 288 and 290 on substrate 232. Phototransistor 234 is seen to
be mounted on a conductive path 292, coupled to metallic pad 290.
Phototransistor 234 is coupled by means of a fine wire 294 to a
second conductive path 296, coupled to metallic pad 288. Conductive
path 292 extends under wall 240, and serves as the mounting point
for infrared LED 236. A third conductive path 298 serves as a
mounting point for red LED 238. LED's 236 and 238 are connected to
pads 292 and 298, respectively, by means of fine wires 300 and 302.
Conductive path 298 is coupled to a fourth conductive path 304 by
means of a fine wire 306 and is coupled to sensor body 230 by
metallic strap 239. All other elements correspond to identically
numbered elements in FIGS. 3, 4 and 5.
FIG. 7 is a functional diagram of the sensor and its associated
circuitry. In this view, the sensor hybrid 400 is illustrated
schematically, showing the interconnection of infrared LED 236, red
LED 238 and phototransistor 234. The three lines 402, 404 and 406
correspond to coiled conductors 272, 274 and 276, respectively, in
FIGS. 5 and 6. Line 408 corresponds to conductor 262, shown in FIG.
5 and electrode 410 corresponds to tip electrode 210, shown in FIG.
2.
The diode pair 236 and 238 is driven by a pair of push pull
amplifiers 412 and 414. Amplifier 412 operates in a voltage output
mode, and amplifier 414 operates in a current output mode. The
returning current from phototransistor 234 is converted to a
proportional DC voltage by the current mirror 416 and is delivered
to the sample and hold switches 418 and 420 which recover the peak
signal for each color. Pacing is accomplished by placing line 402
from voltage driver 412 at a logic 0 and placing line 458 to return
amplifier 422 at a logic 1. For example, if the system employs a
3.6 volt lithium thionylchloride cell, this produces a pacing
voltage of approximately 3.1 volts across the heart. Pacing return
amplifier 422 is coupled to electrode 424, which may conveniently
be the metal enclosure of the implanted pacemaker. Timing for the
entire system is provided by integrated circuitry included in the
timing block 426, which controls the timing and function of the
pacing voltage driver 412 and the current driver 414 as well as
determining times for operation of the sample and hold circuits 418
and 420. The outputs of sample and hold circuits 418 and 420 are
coupled to a division network 428 which divides the output of IR
sample and hold 420, on line 430 by the output of red sample and
hold 418 on line 432 to produce a D.C. voltage signal on line 434
indicative of the percentage of oxygen saturation. IR/R division
network 428 may be of the type illustrated in FIG. 1 of U.S. Pat.
No. 4,202,339 issued to Wirtzfeld et al and incorporated herein in
its entirety. In particular, it may correspond to the division
network labeled 16, and described in column 3, lines 30-36. Line
434, carrying an analog signal indicative of oxygen saturation may
be coupled to the sensor processing circuitry of a pacemaker such
as illustrated in FIG. 1. The sensor and associated circuitry may
replace the sensor 12 and line 14 of FIG. 1, with the output of
IR/R division network 428 functionally coupled to sensor processing
circuitry 30.
The functional interconnection of the circuitry of FIG. 7 with the
remainder of the circuitry of an implantable cardiac pacemaker is
also described in conjunction with the prior art pacemaker
illustrated in FIG. 1. Sensor operation under control of timing
circuit 426 may conveniently be initiated in response to a signal
on line 448 indicative of the occurrence of a cardiac pacing pulse
or a signal on line 438 indicative of a sensed spontaneous
contraction of the heart. The input to timing 426 on line 448
therefore might be functionally coupled to the output of demand
logic 106 on line 34 and the input to timing 426 on line 438 might
be functionally coupled to the output of sense amplifier 102 on
line 108. Return pacing amp 422 may be functionally coupled to
demand logic 106 so as to be activated by a signal on line 448
indicative of time out of the escape interval. Finally, electrode
410 may be coupled via line 442 to the input sense amp 102 on line
104 so that electrode 410 also acts as an EKG sensing
electrode.
FIG. 8 is a schematic of a timing circuit for use with the sensor
lead of FIGS. 2-6. Decade counter 462 serves to time the
application of current to the infrared and red LEDs in the sensor
capsule as well as defining sampling periods during the operation
of the red and infrared LEDs. The basic operation is as follows:
Following a signal on line 438 or 448 indicative of a sensed
contraction of the heart or a delivered pacemaker pulse, set-reset
flip-flop 466 is reset via NOR gate 468, which removes the reset
from counter 462, allowing it to be clocked by 10Khz clock 460, and
driving the signal on IR line 454 to a logic 1 via AND gate 472.
When the signal on line 454 is high, current is applied to activate
the infrared LED 236. During counts 1, 2 and 3 of decade counter
462, the IR SAMPLE line 444 is driven to a logic 1 via NAND gate
476. During counts 1, 2 and 3, the IR sample and hold circuitry is
activated. At a count of 5, the CARRY output of decade counter 462
goes to a logic 0, driving the signal on the RED line 452 to logic
0, and also driving the signal on IR line 454 to a logic 0 via AND
gate 472. While the signal on RED line 452 is at a logic 0, current
is applied to drive the red LED. During counts 6, 7 and 8 of decade
counter 462, the RED SAMPLE line 446 goes to a logic 1, via OR gate
482. During counts 6,7 and 8, the red sample and hold circuitry is
activated. On a count of 9, the set reset flip-flop 466 is reset,
locking decade counter 462 on reset and driving RED line 452 high,
ending delivery of current to the red LED. Thus, this circuitry
provides timing for driving the infrared and red LEDs in sequence,
and sampling each diode during the center portion of the period
during which they are driven. This sequence of events occurs a
fixed time after either a pulse signal on line 448 or a sense
signal on line 438, which assures that the oxygen level is sampled
once per each heartbeat, without running the risk of attempting to
sense the oxygen level during a pacing pulse or resetting the pulse
generator timing in response to electrical currents driven through
the sensor.
FIG. 9 shows a schematic of the analog portions of the circuitry
illustrated in FIG. 7, including the current mirror 416, the
current driver 414, the voltage/pacing driver 412, the pacing
return amp 422 and the red and IR sample and hold circuitry 418 and
420, respectively. The current and voltage regulator circuits used
in the current driver 414 and voltage pacing driver 412 are based
upon a common design often found in audio power amplifier output
stages. The operation of the circuitry in FIG. 9 is best understood
in conjunction with the timing diagram of FIG. 10. In the following
description, all reference numbers of 600 or higher refer to FIG.
10. Starting with a pace signal 601 indicating time out of the
escape interval of the pacemaker, on line 448, the pacing return
amp 422 is activated. The pace signal 600 applies a current across
LED 542 via resistor 544, which activates phototransistor 552 which
turns on transistor 548 driving PACE RETURN line 458 high at 604
and simultaneously driving PACE DRIVE line 402 low via transistor
540, allowing discharge of output capacitor 554 through transistor
548, PACE RETURN line 458, electrode 424 (FIG. 7), electrode 420
(FIG. 7), PACE DRIVE line 402 and transistor 540. In addition, as
discussed in conjunction with FIG. 8, above, the pace signal 600
also takes the reset off decade counter 462 which begins the timing
of the sensing cycle. After the pace signal 600, the Q output of
flip-flop 466 (FIG. 8) goes high, driving IR line 454 high at 608,
turning on transistors 536 and 540, driving PACE/DRIVE line 402 low
at 610 and turning on transistors 512 and 508 providing current
flow through the infrared diode 236 (FIG. 7) of approximately 20
milliamps at 614. COMMON line 404 goes to one LED drop above ground
at 612. This in turn triggers current flow 616 through
phototransistor 234 (FIG. 7) and which, via RETURN line 406 is
applied to current mirror 416. The current mirror comprising
transistors 500, 502 and resistor 504 generates an output signal on
line 456. During counts 2-4 of decade counter 462 (FIG. 8), the IR
SAMPLE line 444 goes high at 618, allowing the output of current
mirror 416 on line 456 to be sampled by the IR sample and hold
circuitry 418. The IR sample and hold circuitry consists of an
analog switch 558 which, when activated by a signal on IR SAMPLE
line 444 passes the voltage on line 456 to op amp 568 and capacitor
562. The sample and hold system does not operate in the classical
sense of sampling until the hold capacitor 562 reaches the exact
input voltage. Instead, the sample time is only a small fraction of
the time constant of the source resistance and the holding
capacitor 562. In this way, in response to any abrupt voltage
swings on line 456, the capacitor 562 swings only part way to the
final value during each sample, but after a series of samples will
converge on the actual source voltage value. This mode of operation
is believed appropriate in that the oxygen saturation level changes
between successive samples is small compared to the overall
capacity of capacitor 562. This approach has the advantage that it
minimizes power consumption and circuit complexity.
On the fifth clock cycle counted by decade counter 462 (FIG. 8),
the RED line 452 goes low at 620 and the IR line 454 goes low at
622. This change turns off transistors 540 and 536 in the
voltage/pacing driver 412 and turns on transistors 532 and 534,
sending the PACE/DRIVE line 402 high at 624. In addition,
transistors 508 and 512 are turned off while transistors 518 and
522 are turned on applying LED current of the opposite polarity of
approximately 20 milliamps at 628 across red LED 536. COMMON line
404 goes to one LED drop below battery voltage at 626. Light
reflected from red LED 236 allows current to flow through
phototransistor 234 at 630, which, via RETURN line 406 and current
mirror 416 provides a proportional voltage on line 456. During
counts 6, 7 and 8 of decade counter 462, the RED SAMPLE line 446
goes high at 632, enabling the red sample and hold circuitry 420,
which includes an analog switch 556, capacitor 560 and an op amp
566, which function in a fashion identical to that of the circuitry
discussed in conjunction with the IR sample and hold circuitry 418.
Due to the high impedance of the pacing return amp 422 when
inactive, the voltage on PACE RETURN line 458 follows the voltage
on PACE/DRIVE line 402, preventing current flow between electrode
410 (FIG. 7) and electrode 424 (FIG. 7).
In order for the specific embodiment of circuitry illustrated to
function properly, it is necessary that high efficiency LED's be
used which preferably limit voltage across the LED's to
approximately 1.6 volts. This places COMMON line 404 at
approximately 1.6 volts during operation of LED 236 and at
approximately 2.0 volts during operation of LED 238. This assures
adequate bias for phototransistor 234.
As noted above, the voltage on COMMON line 404 changes at the point
LED 238 is turned on. Because phototransistor 234 has a large
collector-base capacitance, if the voltage across collector and
emitter decreases, transistor 234 will temporarily shut off for a
period long enough to discharge this capacitance. Therefore, the
circuitry herein has been arranged to provide an increase in
voltage, rather than a decrease, at the point the voltage on COMMON
line 404 changes. Transients due to the COMMON line 404 voltage
change do not appear to be significant at light levels generated by
high efficiency LED's as described herein.
The voltages on RED OUT line 432 and IR OUT line 430 are fed to the
IR/R division network 428 to produce an analog DC voltage signal on
line 434 (FIG. 7) indicative of oxygen saturation. This signal on
line 434 is provided to sensor processing circuitry 30 (FIG. 1) and
used to regulate the escape interval of the pacemaker as described
above.
Similarly, after the signal 646 on SENSE line 438, IR line 454 goes
high at 658 causing PACE/DRIVE line 402 to go low at 650 providing
a current of approximately 20 milliamps at 656 across IR diode 538.
COMMON line 402 goes to an LED drop above ground at 654. This
allows a current to flow through photo transistor 534 at 658 which
is sampled while the IR SAMPLE line 444 is high at 660. Halfway
through the sampling cycle at the count of 5 on the decade counter
462 (FIG. 8), RED line 452 goes low at 662, along with IR line 454,
driving PACE/DRIVE line 402 high at 664 and allowing a current of
approximately 20 milliamps at 670 to flow across the red diode 238
(FIG. 7). COMMON line 404 goes to one LED drop below battery
voltage at 666. Light received from red diode 238 (FIG. 7) allows
current to flow through photo transistor 234 (FIG. 7) at 672, which
is sampled while RED SAMPLE line 446 is high, at 674. Again, the
voltage on PACE RETURN line 458 follows the voltage on PACE/DRIVE
line 402, avoiding current flow between tip electrode 410 (FIG. 7)
and electrode 424 (FIG. 7).
The following components were used to construct the circuitry
illustrated in FIGS. 7, 8 and 9:
______________________________________ Resistors Value( )
Transistors Type ______________________________________ 504 10K 500
2N2907 506 5.6K 502 2N2907 510 33 508 2N2907 514 1K 512 2N2907 516
1K 518 2N2222 520 33 522 2N2222 524 5.6K 530 2N2907 526 5.6K 532
2N2907 528 5.6K 536 2N2222 530 27K 540 2N2222 538 27K 542/552
SPX-33 544 330 Opto-Isolator 234 Stanley SPS-201 Phototransistor
______________________________________
______________________________________ Integrated Circuits Types
______________________________________ 462 RCA CD4017 Capacitors
Value(mF) 466 RCA CD4013 554 10 468 RCA CD4071 560 .1 472 RCA
CD4081 562 .1 476 RCA CD4075 Diodes Type 482 RCA CD4075 236 Optron
OPC-123 556 RCA CD4066 IR LED 558 RCA CD4066 238 Stanley HIK 566
Intersil 7621 Red LED 568 Intersil 7621
______________________________________
The particular circuitry disclosed in FIGS. 8 and 9 is optimized
for use with a lithium thionylchloride battery providing a supply
voltage of 3.6 volts. However, functionally similar circuitry
adapted for use with batteries of other voltages is believed to be
within the scope of the present invention. In addition, in those
embodiments in which lower voltage batteries are used, it may be
desirable to incorporate a voltage doubler in the pacing return
amplifier, in order to provide adequate voltage to capture the
heart. The present invention is also believed broad enough to
encompass such embodiments. It is also acknowledged that there are
numerous alternative ways of providing the time periods provided by
the circuitry of FIG. 8, and other analog and/or digital
equivalents of this circuitry are believed to also be within the
scope of the present invention.
* * * * *